The objective of the proposed experimentation is to obtain information on the cellular and molecular aspects of bone development and biological calcification in the embryonic chick limb. A combined morphological (light and electron microscopic) and biochemical approach is being used in analyzing both in vivo development and a limb cell tissue culture system. This tissue culture system is initiated by plating undifferentiated chick limb mesenchymal cells into culture. We have followed the emergence of specific foci of calcium phosphate deposition, the parathyroid hormone stimulation of adenylate cyclase, cAMP levels, alkaline phosphate activity, collagen biosynthesis, and proteoglycan biosynthesis. We propose to continue and expand these studies in order to attempt to provide experimental details of the emergence of the osteogenic phenotypes, to ascertain what factors influence this emergence and to thus provide an understanding of the chemical, cellular and developmental mechanisms which govern the process of bone formation.